At one point hemp was THE commodity like oil is now. Hemp was essential to travel, light, clothing, engineering and nutrition. They way they changed the perception from the most important thing any statesman had to worry about to something that only poor unwashed people bothered with because they were "addicted." It's nuts.
Imagine in the future once industry has moved on from oil and no one really cares about it except for Mexicans making cool hotrods and getting laid. So they ban oil because they're afraid of the Mexican hotrodders getting too much white tail. Blows my mind.
Well the story is long and multifaceted, but from what I remember the most impact on the prohibition in the USA and globally was made by Harry J. Anslinger, who was married to the niece of Andrew W. Mellon, one of the largest owners of woodlands in the US. It had just been discovered at the time that you can produce 4x the amount of paper from hemp on the same acreage when compared to wood.
NB: Yes this is a conspiracy theory and I can't seem to source it right now, so take it with a few grains of salt.
It would also have been back then, but I don't think Mr. Anslinger and Mr. Mellon valued societal impacts higher than their personal business interests, public assertions to the contrary notwithstanding. Consider that Anslinger is the source of the "reefer madness" propaganda campaign that had serious racial implications. Where there are obvious lies there is usually obvious motive, in this case the business interests of his uncle-in-law, specifically the looming cratering of wood prices with this new development.
Again, total conspiracy theory, I don't have sources to provide. Learned this over a decade ago and can't find them anymore.